It is known that gases in air, especially oxygen, are of great industrial importance because of their many applications in very varied technical fields.
At the present time, the cyclic adsorption technique called "PSA" (short for Pressure Swing Adsorption) is widely used for producing these gases.
More specifically, when the gas mixture to be separated is air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture by virtue of a preferential adsorption of at least the nitrogen on a material that preferentially adsorbs nitrogen.
This nitrogen adsorption is achieved by varying the pressure applied in the separation zone containing the adsorbent material.
The oxygen, since it is not adsorbed or only slightly, is recovered at the outlet of the separation zone, with a purity, in general, greater than 90% or even 93% or higher.
More generally, a PSA process for separating a gas mixture comprising a first compound which is preferentially adsorbed on an adsorbent material and a second compound which is adsorbed less preferentially on the adsorbent material than the first compound, for the purpose of producing the second compound, comprises, in a cyclic manner:
a step of preferential adsorption of at least the first compound on the adsorbent material, at an adsorption pressure called the "high pressure", with recovery of at least one portion of the second compound thus produced; PA1 a step of desorption of the first compound thus trapped by the adsorbent, at a desorption pressure, called the "low pressure", which is less than the adsorption pressure; PA1 a step of repressurization of the separation zone comprising the adsorbent, by gradually increasing the pressure from the low pressure to the high pressure. PA1 the intrinsic strength (.alpha.) is greater than 0.15, preferably greater than 0.20; PA1 the intrinsic strength (.alpha.) is less than 1.5, preferably less than 1; PA1 the zeolite is a zeolite having a faujasite structure with a Si/Al ratio of 1 to 1.25, preferably a LSX-type zeolite with a Si/Al ratio of about 1.02.+-.0.02; PA1 the zeolite is exchanged by monovalent, divalent and/or trivalent cations, preferably lithium, potassium, calcium, zinc, copper, aluminium, strontium or nickel cations; PA1 the zeolite contains from 30 to 99% lithium and/or calcium cations; PA1 the zeolite contains from 80 to 99% lithium cations and/or from 0.1 to 10% potassium cations, preferably from 90 to 99% lithium cations; PA1 the stream of gas to be separated comprises oxygen and nitrogen and preferably the stream of gas is a stream of air, the air being, within the context of the present invention, the air contained inside a building or a heated or unheated enclosure, or the outside air, that is to say under atmospheric conditions, taken as it is or optionally pretreated, especially dried; PA1 the first compound is nitrogen and the second compound is oxygen, and a stream of oxygen-rich gas, that is to say comprising, in general, at least 90% oxygen, is produced; PA1 the stream of gas to be separated comprises at least one of the compounds chosen from CO, CO.sub.2, hydrogen and mixtures thereof, and/or at least one other gas more polar than hydrogen; PA1 the high adsorption pressure is between 10.sup.5 Pa and 10.sup.7 Pa, preferably about 10.sup.5 Pa to 10.sup.6 Pa, and/or the low desorption pressure is between 10.sup.4 Pa and 10.sup.6 Pa, preferably about 10.sup.4 Pa to 10.sup.5 Pa. PA1 the feed temperature is between 10.degree. C. and 80.degree. C., preferably between 25.degree. C. and 60.degree. C.
However, it is known that the efficiency with which a gas mixture, such as air, is separated depends on many parameters, especially the high pressure, the low pressure, the type of adsorbent material used and the affinity of the latter for the compounds to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size and shape of the particles of adsorbent, the composition of these particles and the temperature range being established within the bed of adsorbent.
At the present time, although no general law governing the behaviour has been able to be determined, knowing that it is very difficult to link these various parameters together, it is also known that the nature and the properties of the adsorbent play a paramount role in the overall efficiency of PSA-type processes.
At the present time, zeolites, especially of the A, X, Y or LSX (Low Silica X) type, are the adsorbents most commonly employed in PSA processes.
The zeolite particles usually contain monovalent, divalent and/or trivalent cations, for example cations of alkali metals, of alkaline-earth metals or of lanthanides, these cations being incorporated during the synthesis of the zeolite particles and/or inserted subsequently using an ion-exchange technique as described in the prior art.
Conventionally, the adsorbents most used for the separation of gases, particularly air, are zeolites which are highly exchanged, generally to more than 80%, with metal cations, such as lithium, calcium, strontium, barium, aluminium, zinc or copper cations. In this regard, mention may be made, as examples, of documents U.S. Pat. No. 5,268,023, U.S. Pat. No. 5,174,979, U.S. Pat. No. 4,859,217, U.S. Pat. No. 5,152,813, U.S. Pat. No. 4,481,018, U.S. Pat. No. 5,419,891, EP-A-589,406 and EP-A-589,391.
However, the productivity of a PSA unit depends also on other parameters, especially such as the cycle time.
This is because, in order to increase the productivity of a PSA unit with one or more adsorbers, that is to say to produce more gas, for example oxygen, during a given time period, the cycle time of each of the adsorbers of the unit must be reduced.
To do this, it is necessary to use an adsorbent having sufficiently rapid adsorption kinetics, as described by the document EP-A-785,020. Thus, in order to improve the adsorben: particle kinetics, it is known to be desirable to reduce their hydraulic diameter, which in turn causes, moreover, a pressure drop within the beds of adsorbents to be increased.
Now, reducing the hydraulic diameter of the particles also results in a reduction in their mechanical strength.
Thus, since the adsorbent particles are more brittle they are crushed more easily.
Furthermore, if the strength of the adsorbent particles is too high, deterioration of the kinetics of the molecular sieve may occur.